1. Field of the Invention
This invention relates to a control system and method and an engine control unit for a compression ignition internal combustion engine, for causing a mixture supplied to the engine to burn within a combustion chamber by self-ignition.
2. Prior Art
Conventionally, this kind of control system for a compression ignition internal combustion engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 10-266878. In this control system, low-load to intermediate-load operating regions of the engine are set to a region for executing compression ignition. Further, the timing for opening and closing intake valves and exhaust valves is configured to be variable. When load on the engine is in the region for executing compression ignition, the timing for opening and closing the intake valves and the exhaust valves is controlled according to the load on the engine so as to make self-ignition easy to occur. Particularly, in a low-load operating region, as the load on the engine is lower, that is, as self-ignition becomes more difficult to occur due to the reduced amount of fuel supply, the timing for closing the intake valves is advanced whereby the compression ratio of intake air is increased, and the timing for closing the exhaust valves is advanced whereby part of combustion gas is retained in the combustion chambers. This increases the temperature of a mixture supplied to the combustion chambers to place the mixture in a high temperature condition, thereby making self-ignition easy to occur.
However, according to the conventional control system for a compression ignition internal combustion engine, the increase in the compression ratio and the amount of residual combustion gas that can be retained in each combustion chamber are limited. This is because an extremely high compression ratio causes knocking, and when the amount of residual combustion gas becomes too large, it is impossible to take a sufficient amount of the mixture into the combustion chamber, which prevents a required power output from being obtained from the engine. Therefore, an increase in the mixture temperature by combustion gas is limited, and therefore, self-ignition cannot be caused on the low-load side. As a result, the region for executing the compression ignition is limited on the low-load side.
It is an object of the invention to provide a control system and method and an engine control unit for a compression ignition internal combustion engine, which are capable of expanding a region for executing compression ignition on the low-load side, while positively obtaining a required power output from the engine.
To attain the above object, according to a first aspect of the invention, there is provided a control system for a compression ignition internal combustion engine, for causing a mixture supplied to the engine to burn within a combustion chamber by self-ignition, comprising:
operating condition-detecting means for detecting operating conditions of the engine;
residual combustion gas amount-determining means for determining an amount of residual combustion gas depending on the detected operating conditions of the engine;
combustion gas retention means for retaining part of combustion gas in the combustion chamber after combustion, based on the determined amount of residual combustion gas;
supercharging means for supercharging fresh air flowing to the combustion chamber;
supercharging execution-judging means for judging, based on the detected operating conditions of the engine, whether or not the supercharging by the supercharging means should be executed so as to cause self-ignition; and
supercharging control means for causing the supercharging means to supercharge the fresh air flowing to the combustion chamber when it is judged by the supercharging execution-judging means that the supercharging should be executed.
According to this control system, the amount of residual combustion gas to be retained in the combustion chamber is determined depending on the detected operating conditions of the engine, and the amount of residual combustion gas thus determined is retained in the combustion chamber. Therefore, for instance, if the amount of residual combustion gas is determined depending on required load on the engine, it is possible to retain a sufficient amount of combustion gas within the combustion chamber, thereby making it possible to sufficiently raise the temperature of the mixture to be burnt in the combustion chamber. Further, it is judged, based on the detected operating conditions of the engine, whether or not supercharging of fresh air flowing to the combustion chamber should be executed so as to cause self-ignition, and when it is judged that the supercharging should be executed, the fresh air is supercharged. As a result, to prevent fresh air supplied to the combustion chamber from becoming short e.g. due to a relatively large amount of residual combustion gas, the supercharging of fresh air is executed to cause self-ignition, whereby it is possible to positively supply an amount of fresh air required for self-ignition. Thus, it is possible to ensure the supply of the required amounts of residual combustion gas and fresh air, to cause self-ignition, depending on the operating conditions of the engine, whereby the region for executing compression ignition can be expanded on the low-load side while ensuring that the engine generates a required power output.
Preferably, the supercharging execution-judging means judges that the supercharging by the supercharging means should be executed when the determined amount of residual combustion gas is equal to or larger than a predetermined value.
According to this preferred embodiment, when the amount of residual combustion gas is equal to or larger than the predetermined value, i.e. when fresh air supplied to the combustion chamber becomes short, supercharging is carried out, and hence it is possible to ensure the supply of the required amount of fresh air for self-ignition.
Preferably, the supercharging control means sets supercharging pressure created by the supercharging means to a larger value as the determined amount of residual combustion gas is larger.
According to this preferred embodiment, the supercharging pressure for supercharging fresh air flowing to the combustion chamber is set to a larger value as the amount of residual combustion gas is larger, i.e. as the degree of shortage of fresh air supplied to the combustion chamber is higher, so that it is possible to appropriately supply an amount of fresh air required for self-ignition according to the amount of residual combustion gas.
Preferably, the operating condition-detecting means includes engine speed-detecting means for detecting engine rotational speed and accelerator opening-detecting means for detecting accelerator opening, and the supercharging execution-judging means judges whether or not the supercharging by the supercharging means should be executed, based on a map in which predetermined values of the amount of residual combustion gas are set with respect to required load on the engine dependent on the accelerator opening and the engine rotational speed, and the engine rotational speed.
Preferably, the engine includes an exhaust valve, and the combustion gas retention means retains the part of combustion gas in the combustion chamber by advancing valve-closing timing of the exhaust valve.
Preferably, the engine includes a plurality of exhaust valves, and the combustion gas retention means retains the part of combustion gas in the combustion chamber by opening at least one of the exhaust valves during an intake stroke to thereby draw exhaust gases into the combustion chamber.
To attain the above object, according to a second aspect of the invention, there is provided a method of controlling a compression ignition internal combustion engine, to cause a mixture supplied to the engine to burn within a combustion chamber by self-ignition, comprising:
a detecting step of detecting operating conditions of the engine;
a determining step of determining an amount of residual combustion gas depending on the detected operating conditions of the engine;
a retaining step of retaining part of combustion gas in the combustion chamber after combustion, based on the determined amount of residual combustion gas;
a judging step of judging, based on the detected operating conditions of the engine, whether or not supercharging of fresh air flowing to the combustion chamber should be executed so as to cause self-ignition; and
a control step of providing control such that the supercharging is executed when it is judged in the judging step that the supercharging should be executed.
According to this method of controlling a compression ignition internal combustion engine, the same advantageous effects as provided by the first embodiment of the present invention can be obtained.
Preferably, the judging step includes judging that the supercharging should be executed when the determined amount of residual combustion gas is equal to or larger than a predetermined value.
Preferably, the control step includes setting supercharging pressure to a larger value as the determined amount of residual combustion gas is larger.
Preferably, the detecting step includes detecting engine rotational speed and accelerator opening, and the judging step includes judging whether or not the supercharging should be executed, based on a map in which predetermined values of the amount of residual combustion gas are set with respect to required load on the engine dependent on the accelerator opening and the engine rotational speed, and the engine rotational speed.
Preferably, the engine includes an exhaust valve, and the retention step includes retaining the part of combustion gas in the combustion chamber by advancing valve-closing timing of the exhaust valve.
Preferably, the engine includes a plurality of exhaust valves, and the retention step includes retaining the part of combustion gas in the combustion chamber by opening at least one of the exhaust valves during an intake stroke to thereby draw exhaust gases into the combustion chamber.
According to the above preferred embodiments of the second aspect of the invention, the same advantageous effects as provided by respective corresponding ones of the preferred embodiments of the first aspect of the invention can be obtained.
To attain the above object, according to a third aspect of the invention, there is provided an engine control unit including a control program for causing a computer to control a compression ignition internal combustion engine, to cause a mixture supplied to the engine to burn within a combustion chamber by self-ignition,
wherein the control program causes the computer to detect operating conditions of the engine, determine an amount of residual combustion gas depending on the detected operating conditions of the engine, cause part of combustion gas to be retained in the combustion chamber after combustion, based on the determined amount of residual combustion gas, judge, based on the detected operating conditions of the engine, whether or not supercharging of fresh air flowing to the combustion chamber should be executed so as to cause self-ignition, and provide control such that the supercharging is executed when it is judged that the supercharging should be executed.
According to this engine control unit, the same advantageous effects as provided by the first embodiment of the present invention can be obtained.
Preferably, when the control program causes the computer to judge whether or not the supercharging should be executed, the control program causes the computer to judge that the supercharging should be executed when the determined amount of residual combustion gas is equal to or larger than a predetermined value.
Preferably, when the control program causes the computer to provide control such that the supercharging is executed, the control program causes the computer to set supercharging pressure to a larger value as the determined amount of residual combustion gas is larger.
Preferably, when the control program causes the computer to detect operating conditions of the engine, the control program causes the computer to detect engine rotational speed and accelerator opening, and the control program causes the computer to judge whether or not the supercharging should be executed, based on a map in which predetermined values of the amount of residual combustion gas are set with respect to required load on the engine dependent on the accelerator opening and the engine rotational speed, and the engine rotational speed.
Preferably, the engine includes an exhaust valve, and the control program causes the computer to cause the part of combustion gas to be retained in the combustion chamber by advancing valve-closing timing of the exhaust valve.
Preferably, the engine includes a plurality of exhaust valves, and the control program causes the computer to cause the part of combustion gas to be retained in the combustion chamber by opening at least one of the exhaust valves during an intake stroke to thereby draw exhaust gases into the combustion chamber.
According to the above preferred embodiments of the third aspect of the invention, the same advantageous effects as provided by respective corresponding ones of the preferred embodiments of the first aspect of the invention can be obtained.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.